I imagine it's neither. It is the psychological aspect of the violation. Just as why many women dread vaginal exams, or even having their breasts examined. Necessary but clearly undesirable. I think it may actually be more violative for the average gay man compared to the average straight man, since it's probably more closely tied up with his concept of intimacy.

I was thinking more in terms of it coming from the same place as dudes who refuse to wash their butt because it's 'gay'.

Mammograms hurt a like shit for a lot of women. People aren't just upset about this because of the violation. There is real actual physical intense pain associated with these procedures.

This is to say nothing about various gynecological exams. "Let me scrape your cervix with a metal tool" is a pretty common procedure, for example.